Optical readers, such as bar code readers, are now quite common. Typically, a bar code comprises a series of encoded symbols, and each symbol consists of a series of light and dark regions, typically in the form of rectangles. The widths of the dark regions, the bars, and/or the widths of the light spaces between the bars indicate the encoded information.
A bar code reader illuminates the code and senses light reflected from the code to detect the widths and spacings of the code symbols and derive the encoded data. Bar code reading type data input systems improve the efficiency and accuracy of data input for a wide variety of applications. The ease of data input in such systems facilitates more frequent and detailed data input, for example to provide efficient inventories, tracking of work in progress, etc.
A variety of scanning devices are known. One particularly advantageous type of reader is an optical scanner which scans a beam of light, such as a laser beam, across the symbols. Laser scanner systems and components of the type exemplified by U.S. Pat. Nos. 4,387,297 and 4,760,248--which are owned by the assignee of the instant invention and are incorporated by reference herein--have generally been designed to read indicia having parts of different light reflectivity, e.g., bar code symbols, particularly of the Universal Product Code (UPC) type, at a certain working range or reading distance from a hand-held or stationary scanner.
FIG. 1 illustrates an example of a prior art bar code reader unit 10 implemented as a gun shaped device, having a pistol-grip type of handle 53. A lightweight plastic housing 55 contains the laser light source 46, the detector 58, the optics and signal processing circuitry and the CPU 40, as well as a power source or battery 62. A light-transmissive window 56 in the front end of the housing 55 allows the outgoing light beam 51 to exit and the incoming reflected light 52 to enter. The reader 10 is designed to be aimed at a bar code symbol 70 by the user from a position in which the reader 10 is spaced from the symbol, i.e., not touching the symbol or moving across the symbol.
As further depicted in FIG. 1, a suitable lens 57 (or multiple lens system) may be used to focus the scanned beam into a scanning spot at an appropriate reference plane. A light source 46, such as a semiconductor laser diode, introduces a light beam into the axis of the lens 57, and the beam passes through a partially-silvered mirror 47 and other lenses or beam-shaping structure as needed. The beam is reflected from an oscillating mirror 59 which is coupled to a scanning motor 60 energized when the trigger 54 is pulled. The oscillation of the mirror 59 causes the reflected beam 51 to scan back and forth in a desired pattern.
A variety of mirror and motor configurations can be used to move the beam in a desired scanning pattern. For example, U.S. Pat. No. 4,251,798 discloses a rotating polygon having a planar mirror at each side, each mirror tracing a scan line across the symbol. U.S. Pat. Nos. 4,387,297 and 4,409,470 both employ a planar mirror which is repetitively and reciprocally driven in alternate circumferential directions about a drive shaft on which the mirror is mounted. U.S. Pat. No. 4,816,660 discloses a multi-mirror construction composed of a generally concave mirror portion and a generally planar mirror portion. The multi-mirror construction is repetitively reciprocally driven in alternate circumferential directions about a drive shaft on which the multi-mirror construction is mounted.
The light 52 reflected back by the symbol 70 passes back through the window 56 for application to the detector 58. In the exemplary reader 10 shown in FIG. 1, the reflected light reflects off of mirror 59 and partially-silvered mirror 47 and impacts on the light sensitive detector 58. The detector 58 produces an analog signal proportional to the intensity of the reflected light 52.
A digitizer circuit mounted on board 61 processes the analog signal from detector 58 to produce a pulse signal where the widths and spacings between the pulses correspond to the widths of the bars and the spacings between the bars. The digitizer serves as an edge detector or wave shaper circuit, and the threshold value set by the digitizer determines what points of the analog signal represent bar edges. The pulse signal from the digitizer is applied to a decoder, typically a programmed microprocessor 40. Typically, the microprocessor decoder 40 will have associated program memory and random access data memory. The decoder first determines the pulse widths and spacings of the signal from the digitizer. The decoder then analyzes the widths and spacings to find and decode a legitimate bar code message. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard. This may also include an initial recognition of the particular standard the scanned symbol conforms to. This recognition of the standard is typically referred to as autodiscrimination.
To scan a symbol 70, a user aims the bar code reader unit 10 and operates movable trigger switch 54 to activate the light beam 51, the scanning motor 60 and the detector circuitry. If the scanning beam is visible, the operator can see the scan pattern on the surface on which the symbol appears and adjust aiming of the reader 10 accordingly. If the light produced by the source 46 is marginally visible, an aiming light may be included in the optical system. The aiming light if needed, produces a visible-light spot which may be fixed, or scanned just like the laser beam; the user employs this visible light to aim the reader unit at the symbol before pulling the trigger.
The reader 10 may also function as a portable computer terminal. If so, the bar code reader 10 would include a keyboard 48 and a display 49, such as described in the previously noted U.S. Pat. No. 4,409,470.
In many of the prior art scanners of the type generally discussed above, flexible support means are provided to support one or more of the optical components for reciprocal motion. Although such support structure is not shown separately in FIG. 1, typically the flexible support would support the mirror 59 to permit angular oscillatory motion thereof in response to activation of scanning motor 60. As the size of the scanner is reduced to reduce weight and make prolonged operation more comfortable and convenient, manufacturers have tried to decrease the size of the mirror and its support structure. Many flexible support structures formed of flat strip materials, however, have lacked sufficient physical strength to support the mirror. As a result, the mirror tends to droop in a manner which disrupts the optical alignment of the mirror with the laser source 46.
Also, in some scanning applications it is desirable to scan at extremely low frequencies, for example at or below 20 Hz. This is particularly true in devices which optically scan in two different directions to scan an indicia which includes two or more lines of optically encoded information. To read such two-dimensional codes, a first direction (e.g. an X-direction) is scanned at a relatively high rate, while the second direction (e.g. Y-direction) is scanned at the low rate. This produces a raster or similar two-dimensional scanning pattern having a relatively high density of the X-direction scan lines. Flexible support structures for movement of mirrors or other optical components at such low rates, however, are susceptible to low frequency jitter caused by movement of the hand in which the operator holds the scanner. Hand motion typically induces a noise vibration on the order of 2 Hz to 10 Hz, and such a vibration will cause a vibration of the low speed scanning support mechanism and disrupt the scanning pattern.
Another problem with prior art scanners relates to working range, particularly that of scanners designed to read two-dimensional symbologies. The working range is defined as the region within which the scanning pattern is sufficient to permit accurate decoding as the beam pattern passes across the bar code symbol. A two-dimensional scanning pattern has a finite number of lines in the first (X) direction. When the scanner is close to the scanned surface, the scanning pattern is small, and the lines are close together (high line density). If the operator moves the scanner further away, however, the scan pattern enlarges, and the lines are further apart (lowering line density). Consequently, as the distance between the scanner and the symbol moves out of the working range of the scanner, which is typically only a few inches in length, the density of the scan lines in the two dimensional pattern drops so low that it prevents accurate reading of the two-dimensional bar code. Present two-dimensional scanning systems, accordingly, must be positioned within a relatively narrow range of distances from a symbol in order to properly read the symbol, which may make operation inconvenient and difficult.